Today Dynamic Memory Allocation: Advanced …...Implicit Memory Management: Garbage Collection Garbage collection: automatic reclamation of heap-allocated storage—application never

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1Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition

Dynamic Memory Allocation:Advanced Concepts

CSci 2021: Machine Architecture and OrganizationApril 27th-29th, 2020Your instructor: Stephen McCamant

Based on slides originally by:

Randy Bryant, Dave O’Hallaron


Today

Explicit free lists

Segregated free lists

Garbage collection

Memory-related perils and pitfalls


Keeping Track of Free Blocks

Method 1: Implicit free list using length—links all blocks

Method 2: Explicit free list among the free blocks using pointers

Method 3: Segregated free list Different free lists for different size classes

Method 4: Blocks sorted by size Can use a balanced tree (e.g. Red-Black tree) with pointers within each

free block, and the length used as a key

5 4 26

5 4 26


Explicit Free Lists

Maintain list(s) of free blocks, not all blocks The “next” free block could be anywhere

So we need to store forward/back pointers, not just sizes

Still need boundary tags for coalescing

Luckily we track only free blocks, so we can use payload area

Size

Payload andpadding

a

Size a

Size a

Size a

Next

Prev

Allocated (as before) Free


Explicit Free Lists

Logically:

Physically: blocks can be in any order

A B C

4 4 4 4 66 44 4 4

Forward (next) links

Back (prev) links

A B

C


Allocating From Explicit Free Lists

Before

After

= malloc(…)

(with splitting)

conceptual graphic

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Freeing With Explicit Free Lists

Insertion policy: Where in the free list do you put a newly freed block?

LIFO (last-in-first-out) policy

Insert freed block at the beginning of the free list

Pro: simple and constant time

Con: studies suggest fragmentation is worse than address ordered

Address-ordered policy

Insert freed blocks so that free list blocks are always in address order:

addr(prev) < addr(curr) < addr(next)

Con: requires search

Pro: studies suggest fragmentation is lower than LIFO


Freeing With a LIFO Policy (Case 1)

Insert the freed block at the root of the list

free( )

Root

Root

Before

After

conceptual graphic



Splice out successor block, coalesce both memory blocks and insert the new block at the root of the list

free( )

Root

Before

Root

After

conceptual graphic



Splice out predecessor block, coalesce both memory blocks, and insert the new block at the root of the list

free( )

Root

Root

Before

After

conceptual graphic



Splice out predecessor and successor blocks, coalesce all 3 memory blocks and insert the new block at the root of the list

free( )

Root

Before

Root

After

conceptual graphic


Explicit List Summary

Comparison to implicit list: Allocate is linear time in number of free blocks instead of all blocks

Much faster when most of the memory is full

Slightly more complicated allocate and free since needs to splice blocks in and out of the list

Some extra space for the links (2 extra words needed for each block)

Does this increase internal fragmentation?

Most common use of linked lists is in conjunction with segregated free lists Keep multiple linked lists of different size classes, or possibly for

different types of objects

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Keeping Track of Free Blocks

Method 1: Implicit list using length—links all blocks

Method 2: Explicit list among the free blocks using pointers

Method 3: Segregated free list Different free lists for different size classes

Method 4: Blocks sorted by size Can use a balanced tree (e.g. Red-Black tree) with pointers within each

free block, and the length used as a key

5 4 26

5 4 26


Today

Explicit free lists


Garbage collection



Segregated List (Seglist) Allocators

Each size class of blocks has its own free list

Often have separate classes for each small size

For larger sizes: One class for each two-power size

1-2

3

4

5-8

9-inf


Seglist Allocator

Given an array of free lists, each one for some size class

To allocate a block of size n: Search appropriate free list for block of size m > n

If an appropriate block is found:

Split block and place fragment on appropriate list (optional)

If no block is found, try next larger class

Repeat until block is found

If no block is found: Request additional heap memory from OS (using sbrk())

Allocate block of n bytes from this new memory

Place remainder as a single free block in largest size class.


Seglist Allocator (cont.)

To free a block: Coalesce and place on appropriate list

Advantages of seglist allocators Higher throughput

log time for power-of-two size classes

Better memory utilization

First-fit search of segregated free list approximates a best-fit

search of entire heap.

Extreme case: Giving each block its own size class is equivalent to

best-fit.


More Info on Allocators

D. Knuth, “The Art of Computer Programming”, 2nd edition, Addison Wesley, 1973 The classic reference on dynamic storage allocation

Wilson et al, “Dynamic Storage Allocation: A Survey and Critical Review”, Proc. 1995 Int’l Workshop on Memory Management, Kinross, Scotland, Sept, 1995.

Comprehensive survey

Available from CS:APP student site (csapp.cs.cmu.edu)

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Today

Explicit free lists


Garbage collection



Implicit Memory Management:Garbage Collection

Garbage collection: automatic reclamation of heap-allocated storage—application never has to free

Common in many dynamic languages: Python, Ruby, Java, Perl, ML, Lisp, Mathematica

Variants (“conservative” garbage collectors) exist for C and C++ However, cannot necessarily collect all garbage

void foo() {

int *p = malloc(128);

return; /* p block is now garbage */

}


Garbage Collection

How does the memory manager know when memory can be freed? In general we cannot know what is going to be used in the future since it

depends on conditionals

But we can tell that certain blocks cannot be used if there are no pointers to them

Must make certain assumptions about pointers Memory manager can distinguish pointers from non-pointers

All pointers point to the start of a block

Cannot hide pointers (e.g., by coercing them to an int, and then back again)


Classical GC Algorithms

Mark-and-sweep collection (McCarthy, 1960) Does not move blocks (unless you also “compact”)

Reference counting (Collins, 1960) Does not move blocks (not discussed)

Copying collection (Minsky, 1963)

Moves blocks (not discussed)

Generational Collectors (Lieberman and Hewitt, 1983)

Collection based on lifetimes

Most allocations become garbage very soon

So focus reclamation work on zones of memory recently allocated

For more information: Jones and Lin, “Garbage Collection: Algorithms for Automatic Dynamic Memory”, John Wiley & Sons, 1996.


Memory as a Graph We view memory as a directed graph

Each block is a node in the graph

Each pointer is an edge in the graph

Locations not in the heap that contain pointers into the heap are called root nodes (e.g. registers, locations on the stack, global variables)

Root nodes

Heap nodes

Not-reachable(garbage)

reachable

A node (block) is reachable if there is a path from any root to that node.

Non-reachable nodes are garbage (cannot be needed by the application)25Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition

Mark and Sweep Collecting

Can build on top of malloc/free package Allocate using malloc until you “run out of space”

When out of space: Use extra mark bit in the head of each block

Mark: Start at roots and set mark bit on each reachable block

Sweep: Scan all blocks and free blocks that are not marked

After mark Mark bit set

After sweep freefree

root

Before mark

Note: arrows here denote

memory refs, not free list ptrs.

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Assumptions For a Simple Implementation

Application new(n): returns pointer to new block with all locations cleared

read(b,i): read location i of block b into register

write(b,i,v): write v into location i of block b

Each block will have a header word addressed as b[-1], for a block b

Used for different purposes in different collectors

Instructions used by the Garbage Collector is_ptr(p): determines whether p is a pointer

length(b): returns the length of block b, not including the header

get_roots(): returns all the roots


Mark and Sweep (cont.)

ptr mark(ptr p) {

if (!is_ptr(p)) return; // do nothing if not pointer

if (markBitSet(p)) return; // check if already marked

setMarkBit(p); // set the mark bit

for (i=0; i < length(p); i++) // call mark on all words

mark(p[i]); // in the block

return;

}

Mark using depth-first traversal of the memory graph

Sweep using lengths to find next block

ptr sweep(ptr p, ptr end) {

while (p < end) {

if markBitSet(p)

clearMarkBit();

else if (allocateBitSet(p))

free(p);

p += length(p);

} }


Conservative Mark & Sweep in C

A “conservative garbage collector” for C programs is_ptr() determines if a word is a pointer by checking if it points to

an allocated block of memory (might also be an integer with same val.)

But, in C pointers can point to the middle of a block

So how to find the beginning of the block? Can use a balanced binary tree to keep track of all allocated blocks (key

is start-of-block)

Balanced-tree pointers can be stored in header (use two additional words)

Header

ptr

Head Data

Left Right

SizeLeft: smaller addressesRight: larger addresses


Today

Explicit free lists


Garbage collection



Memory-Related Perils and Pitfalls

Dereferencing bad pointers

Reading uninitialized memory

Overwriting memory

Referencing nonexistent variables

Freeing blocks multiple times

Referencing freed blocks

Failing to free blocks


C operatorsOperators Associativity() [] -> . left to right! ~ ++ -- + - * & (type) sizeof right to left* / % left to right+ - left to right<< >> left to right< <= > >= left to right== != left to right& left to right^ left to right| left to right&& left to right|| left to right?: right to left= += -= *= /= %= &= ^= != <<= >>= right to left, left to right

->, (), and [] have high precedence, with * and & just below

Unary +, -, and * have higher precedence than binary forms

Source: K&R page 53

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C Pointer Declarations: Test Yourself!int *p

int *p[13]

int *(p[13])

int **p

int (*p)[13]

int *f()

int (*f)()

int (*(*f())[13])()

int (*(*x[3])())[5]

p is a pointer to int

p is an array[13] of pointer to int

p is an array[13] of pointer to int

p is a pointer to a pointer to an int

p is a pointer to an array[13] of int

f is a function returning a pointer to int

f is a pointer to a function returning int

f is a function returning ptr to an array[13]of pointers to functions returning int

x is an array[3] of pointers to functions returning pointers to array[5] of ints

Source: K&R Sec 5.1233Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition

Dereferencing Bad Pointers

The classic scanf bug

int val;

...

scanf(“%d”, val);


Reading Uninitialized Memory

Assuming that heap data is initialized to zero

/* return y = Ax */

int *matvec(int **A, int *x) {

int *y = malloc(N*sizeof(int));

int i, j;

for (i=0; i<N; i++)

for (j=0; j<N; j++)

y[i] += A[i][j]*x[j];

return y;

}


Overwriting Memory

Allocating the (possibly) wrong sized object

int **p;

p = malloc(N*sizeof(int));

for (i=0; i<N; i++) {

p[i] = malloc(M*sizeof(int));

}


Overwriting Memory

Off-by-one error

int **p;

p = malloc(N*sizeof(int *));

for (i=0; i<=N; i++) {

p[i] = malloc(M*sizeof(int));

}


Overwriting Memory Not checking the max string size

Basis for classic buffer overflow attacks

char s[8];

int i;

gets(s); /* reads “123456789” from stdin */

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Overwriting Memory

Misunderstanding pointer arithmetic

int *search(int *p, int val) {

while (*p && *p != val)

p += sizeof(int);

return p;

}


Overwriting Memory

Referencing a pointer instead of the object it points to

*size-- is *(size--), you meant (*size)--

int *BinheapDelete(int **binheap, int *size) {

int *packet;

packet = binheap[0];

binheap[0] = binheap[*size - 1];

*size--;

Heapify(binheap, *size, 0);

return(packet);

}


Referencing Nonexistent Variables

Forgetting that local variables disappear when a function returns

int *foo () {

int val;

return &val;

}


Freeing Blocks Multiple Times

Nasty!

x = malloc(N*sizeof(int));

<manipulate x>

free(x);

y = malloc(M*sizeof(int));

<manipulate y>

free(x);


Referencing Freed Blocks

Evil!

x = malloc(N*sizeof(int));

<manipulate x>

free(x);

...

y = malloc(M*sizeof(int));

for (i=0; i<M; i++)

y[i] = x[i]++;


Failing to Free Blocks (Memory Leaks)

Slow, long-term killer!

foo() {

int *x = malloc(N*sizeof(int));

...

return;

}

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Failing to Free Blocks (Memory Leaks)

Freeing only part of a data structure

struct list {

int val;

struct list *next;

};

foo() {

struct list *head = malloc(sizeof(struct list));

head->val = 0;

head->next = NULL;

<create and manipulate the rest of the list>

...

free(head);

return;

}


Dealing With Memory Bugs Debugger: gdb

Good for finding bad pointer dereferences

Sometimes useful for corruption: watchpoints

Hard to detect the other memory bugs

Data structure consistency checker Runs silently, prints message only on error

Use as a probe to zero in on error

Binary translator: valgrind Powerful debugging and analysis technique

Dynamically rewrites code from executable object file

Checks each individual reference at runtime

Bad pointers, overwrites, refs outside of allocated block

glibc malloc contains checking code setenv MALLOC_CHECK_ 3

Documents

Today Dynamic Memory Allocation: Advanced …...Implicit Memory Management: Garbage Collection Garbage collection: automatic reclamation of heap-allocated storage—application never